Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 060859

National Weather Service Sioux Falls SD
359 AM CDT Thu Aug 6 2020

.SHORT TERM...(Today and Tonight)
Issued at 359 AM CDT Thu Aug 6 2020

There is some patchy fog west of the James River Valley, which will
linger around until eroded by the morning sunlight. After any
lingering overnight showers and isolated thunderstorms exit off to
our east, mostly cloudy conditions will continue for areas in NW IA
and SW MN, while areas west of I-29 will be under mostly sunny
skies. This will have a big effect on afternoon temperatures, as
those under cloud coverage likely won`t break into the 80s whereas
those under sunny skies will get into the mid 80s. The slow moving
upper level wave causing the cloudy conditions, in combination with
warm air advection on the 850mb surface, could create scattered
showers and isolated thunderstorms in the early afternoon into the
evening hours. These storms will be quite elevated with the LFC not
being reached until 700mb, and with their unidirectional wind
profile gives confidence in saying these storms will remain well
below severe criteria.

Ridging builds aloft during the evening hours which further pushes
off the trough to the east. This would normally lead to a quiet
night, but numerous HREF ensemble members show a group of
thunderstorms triggering off a shortwave in the early evening hours
along the Nebraska/South Dakota border near the Black Hills. SBCAPE
values drop off during the evening as the storms travel eastwards,
but MUCAPE remains strong at around 2KJ/KG with shear values in the
20-30 knot range. This means that while the storms will most likely
be elevated by the time they reach the MO River valley in central
SD, they could remain strong enough to produce severe weather. SPC`s
day 1 marginal outlook covers this threat well, with the biggest
risk being strong damaging winds. Thunderstorms will remain possible
through the overnight hours, possibly lingering for a couple hours
after sunrise.

.LONG TERM...(Friday through Wednesday)
Issued at 359 AM CDT Thu Aug 6 2020

Friday through Sunday will be categorized as warm and breezy, with a
couple chances for thunderstorms. As the upper level ridge begins to
flatten, an approaching boundary from the west begins to increase
the surface pressure gradient across the area. This will cause
breezy southerly flow, bringing warm and moist air into the region.
Fridays afternoon highs will be in the mid to upper 80s, with heat
indices in the upper 80s to upper 90s courtesy of dewpoints in the
60s. Chances for thunderstorms return in the evening, as another
shortwave passes by to our north, causing storms to form near the
SD/ND border and will likely travel southwards into our region.
There is some question as to whether or not storms will be able to
survive in our strongly capped atmosphere, but the threat for severe
weather is highlighted by SPCs marginal risk for areas north of I-29.

Saturday and Sunday continue the warm moist air being brought into
the region, with heat indices flirting near the 100 degree marker on
both days. Saturday evening could see another round of showers and
thunderstorms as another shortwave passes over the region. This will
likely be another nocturnal event, where MLCAPE values range from 3-
4KJ/KG and shear values approach 40 knots. Certainly an event to
keep an eye on as it remains close to the approaching boundary, and
is well highlighted by SPC`s day 3 marginal and slight risk across
the area.

A high pressure system is progged to move into the area Monday
morning, and will kick out the warm and muggy air by sunrise.
Afternoon high temperatures will be brought back down into the lower
80s, increasing slightly into the mid 80s by the middle of the week.
A couple hints at some precipitation chances near the beginning and
middle of the week, but we shall see.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1100 PM CDT Wed Aug 5 2020

Scattered showers and isolated thunderstorms will be possible
through the overnight hours for northwest Iowa (including KSUX)
and southwest Minnesota. No strong storms are expected, but brief
heavy rain and occasional lightning is possible with the stronger
cells. In southeast South Dakota, showers are not expected, but
clearing skies will allow areas of fog to form around sunrise.
This may be dense in valleys and low spots, but should not be
overly widespread. Fog should dissipate quickly after sunrise with
light southeast winds around 10 kt becoming common during the day.




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